Sunday, January 01, 2012

Book Club: The Origin of Species

I threw my lot in with Professor Angry by Choice to participate in his book club, reading Charles Darwin's The Origin of Species, one chapter at a time. Today marks his deadline for reading chapter 1, which I did over a couple of days during my christmas break.

Oxford "World's Classics" edition, with introduction by Gillian Beer

Oxford University Press, 1996

After the general introduction by Gillian Beer, we get to Darwin's Introduction, which is mostly an explanation of the circumstances surrounding the publication of the first edition of The Origin of Species. This printing follows most closely the second edition, published in early 1860 that included some responses and clarifications by Darwin to some early reactions to the first edition of the book. The important distinction between the fact of evolution, visible in the fossil record, biogeography, and other lines of observational evidence, and the theory of natural selection is made in the Introduction, with an emphasis that the observations alone are intellectually insufficient and require some mechanistic theory to explain them. The Introduction ends with:

I am fully convinced that species are not immutable; but that those belonging to what are called the same genera are lineal descendants of some other and generally extinct species, in the same manner as the acknowledged varieties of any one species are the descentants of that species. Furthermore, I am convinced that Natural Selection has been the main but not exclusive means of modification.

In these two sentences, Darwin lays out the importance of evolution for biological taxonomy and the relationship between descent and modification through time and Linnean taxonomy, as well as the point that natural selection is one of presumably several mechanisms by which this modification can occur. It's right there in the Introduction: natural selection is not the only thing in evolution.

Chapter 1

The part about the "many laws regulating variation" is interesting. Darwin describes several examples of what would now be called pleiotropy, or apparent connections between traits, and mentions the constraint this places on artificial selection - breeders manipulating one trait, such as beak size in pigeons, also carry along correlated changes in other traits, such as foot size. I think the field of evolution-and-development ("evo-devo") has explored and continues to explore this realm.

The following few pages, read from a modern perspective, appear to me as so much weak speculation, given Darwin's ignorance of genetics and embryology. There are some interesting examples there, and it is easy to see how he is tailoring his arguments to his audience, but I find it a little boring.

There's a discussion of the various views of the relationships between domestic and wild species, and the morphological characteristics used to distinguish between species and genera. Apparently, in the mid-1800's there was a popular view that any domesticated species set loose in the wild would automatically revert to its wild form in a few generations, and Darwin devotes some text to showing the holes in this argument - basically, there's no evidence at all to support this hypothesis, and considerable evidence, both circumstantial and direct, that it is wrong. There is also a use of the sneering or dismissive form of the word "empirical", in:

It has often been stated that domestic races do not differ from each other in characters of generic value. I think it could be shown that this statement is hardly correct; but naturalists differ widely in determining what characters of generic value; all such valuations being at present emprical.

The first example (possibly) of Darwin's ideas being later demonstrated incorrect: dogs.

I do not believe, as we shall presently see, that the whole amount of difference between the several breeds of the dog has been produced under domestication; I believe that some small part of the difference is due to their being descended from distinct species.

I'm pretty sure the entire domestic dog diversity arose from the domestication of the Eurasion grey wolf, Canis lupus, followed by strong selection on different lineages, much of it by breeders in the last few hundred years. He does say, however, "the whole amount of difference", and I wouldn't be too surprised by a bit of injection of genetic material from other wolf species (avoiding completely any arguments about species-level diversity within wolves) in the dingo or North American dog lineages.

I am less familiar with the work on the other example domestic animals given by Darwin, horses, cattle, sheep, goats, ducks, rabbits, and chickens, though I suspect all have been demonstrated to be derived from a single wild ancestor species, not many. After some more wanderings among the dogs, we get to Darwin's famous study of pigeons.

On the Breeds of the Domestic Pigeon - Believing that it is always best to study some special group, I have, after deliberation, taken up domestic pigeons.

The structure of the argument for natural selection is becoming clear in Chapter 1. Darwin is using the example of domestic pigeons to lay the groundwork, showing that extreme traits such as beak size or flight characteristics do not have to derive from wild ancestors, and can arise from the careful selection of mating pairs by breeders. This provides the first important piece of evidence, that novel traits can arise from a population and be heritable. I hadn't realized this point needed to be made, but this sub-argument takes the form of a thorough demolishing of the notion that all domestic races must have been each independently derived from now-extinct wild species.

Selection - Let us now briefly consider the steps by which domestic races have been produced, either from one or from several allied species.

This section is a fairly straightforward argument about the visible effects of many generations of Selection (always capitalized in as "Selection" here), both "conscious" as by breeders and "unconsious" by everybody else, on domestic animals and plants. Perhaps it is mostly my 150-years-after-the-fact perspective, but there were few surpises in this part.

1 comment:

Anonymous said...

I nominate Jum Butcher's Dresden Files series for your next book club entry. Shapeshifters is a species I'd love to read your comments on ;) This one's a little over my head, and I have so enjoyed your blog posts in the past.... :)